Going into Manila a long time ago and vectored high (4000 ft due noise abatement) to final ILS. Did the usual check of DME v height and it didn't tie in with the indicated glide slope. At 8 DME we should be close to 2500 ft if on slope. However, at that time we were above a low layer of stratus but at 3500ft due vectors. The ILS showed we were very slightly high on GS (quarter dot) when it should have shown full scale fly down.
We were Vref 40, no wind and on localiser with rate of descent well above normal for three degree slope. Became visual at 1500 ft with GS still showing slightly high but visual scene showed very high and obviously the glide slope we were on was around five degrees. All manual flying in the 737-200. As well, the T-VASIS was a dogs dinner all over the place. Landed and reported apparent false GS to ATC and crook T-VASIS indications as well. ATC advised they were aware of T-VASIS problem and that there had been recent earth tremors which had displaced the T-VASIS boxes. Hence incomprehensible light indications. However ATC SOP required the T-VASIS be switched on for all jet landings (regardless of serviceability!). No NOTAMs of course...
Some months earlier, an Air Manila (?) Boeing 707 had crashed short of the same runway with full flap, idle power and speed brakes extended. The 707 was a write off but minimal casualties I think. The captain swore he was on glide slope all the way from 15 miles out with no wind. Well he was but it was a false glide slope. But to maintain that glide slope he was forced into a high rate of descent about 1800 fpm hence full dirty up and idle thrust. They were fully visual too. Instead of going around, he pressed on regardless to see if he could make it OK. Asked later by investigators why he did not go around when well outside stable approach criteria, he said he thought the noise of the engines spooling up would scare the passengers
Next day, after we reported our episode at Manila, a KingAir flight calibration aircraft tested the ILS and confirmed a flyable glide slope about 5.5 degrees (in other words a false glide slope). That is why the 707 pilot had to dive to fly the false glide slope and hard to believe the captain pressed on. In another life I flew DC3 radio navaid flight calibration aircraft in Australia. Part of the routine ILS test was to start at about 1500 feet on localiser at approx. 5 miles and maintain that height while observing glide slope indication. The test was to look for false glide slopes. It was common to see a false glide slope indication as we got closer to the threshold but these were at 50-70 degrees and of course quite unflyable. We did a similar test for false localiser indications by flying an arc across the localiser at 10 miles. False courses beyond 35 degrees from the localiser were normal and information published in the appropriate pilot documentation. There is no question in my mind that it is good airmanship to always be aware of DME v height cross check (where possible) during any instrument approach as well as a close eye on ground speed readings on short final.
The high angle false glide slopes were simply part of normal ILS glide slope characteristics and as you would need to be a Stuka dive-bomber to fly a 70 degree glide slope, they were of no flight safety concern. I was surprised to read on Pprune that the Eindhoven standard ILS calibration tests did not include a procedure for checking for false glide slopes as we did 50 years ago.